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Abstract

Increased nutrient loading to estuaries has led to
eutrophication, degraded water quality, and ecological transformations.
Quantifying nutrient loads in systems with significant
groundwater input can be difficult due to the
challenge of measuring groundwater fluxes. We quantified
tidal and freshwater fluxes over an 8-week period at the
entrance of West Falmouth Harbor, Massachusetts, a eutrophic,
groundwater-fed estuary. Fluxes were estimated from
velocity and salinity measurements and a total exchange
flow (TEF) methodology. Intermittent cross-sectional measurements
of velocity and salinity were used to convert point
measurements to cross-sectionally averaged values over the
entire deployment (index relationships). The estimated
mean freshwater flux (0.19 m3/s) for the 8-week period
was mainly due to groundwater input (0.21 m3/s) with
contributions from precipitation to the estuary surface
(0.026 m3/s) and removal by evaporation (0.048 m3/s).
Spring–neap variations in freshwater export that appeared
in shorter-term averages were mostly artifacts of the index
relationships. Hydrodynamic modeling with steady groundwater
input demonstrated that while the TEF methodology resolves the freshwater flux signal, calibration of the index–
salinity relationships during spring tide conditions only was
responsible for most of the spring–neap signal. The mean
freshwater flux over the entire period estimated from the
combination of the index-velocity, index–salinity, and TEF
calculations were consistent with the model, suggesting that
this methodology is a reliable way of estimating freshwater
fluxes in the estuary over timescales greater than the spring–
neap cycle. Combining this type of field campaign with
hydrodynamic modeling provides guidance for estimating
both magnitude of groundwater input and estuarine storage
of freshwater and sets the stage for robust estimation of the
nutrient load in groundwater.

Description

This paper is not subject to U.S. copyright. The definitive version was published in Estuaries and Coasts 35 (2012): 1285-1298, doi:10.1007/s12237-012-9515-x.

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